The themes affected are Afterburner, Anomaly, Base, Boxxie, Canvas. Formal White, FormFactor, Leatherbound, Splash and Standard Old.
As Administrator enable 'Allow theme change by URL' in Theme settings.
Go to a page where there are blocks on both sides of the page and turn editing ON
Add &theme=themename (where themename represents one of the affected themes listed above).
Test each theme in turn to see that the ADD BLOCK appears on the LEFT, and not the right as was previously the case.

The main moodle.git repository has just been updated with latest weekly modifications. You may wish to rebase your PULL branches to simplify history and avoid any possible merge conflicts. This would also make integrator's life easier next week.

Eloy Lafuente (stronk7)
added a comment - 19/May/12 7:01 PM The main moodle.git repository has just been updated with latest weekly modifications. You may wish to rebase your PULL branches to simplify history and avoid any possible merge conflicts. This would also make integrator's life easier next week.
TIA and ciao

Sam Hemelryk
added a comment - 28/May/12 10:51 AM Thanks Mary, this has been integrated now.
I did revert your changes on arialist which only has the post-region as well as one other area where there was no pre-region.
Cheers
Sam

Mary Evans
added a comment - 28/May/12 7:20 PM @Martin
Thanks for the vote of confidence with this, it is a simple solution and one which I use all the time.
I'm very pleased it's found its way into Moodle CORE.
Cheers
Mary

Mary Evans
added a comment - 21/Jun/12 1:35 AM I think I missed off some CSS to take account of when you are in side-pre-only situation and want to move blocks to side-post. You can't!
I'll create a new sub-task and link this to it.